The present invention concerns stabilization against color deterioration of filled polyolefin compsoitions during processing. More particularly, the invention relates to reduction of discoloration during processing of polyolefins compositions caused by incorporation of mineral fillers into the polyolefin.
It is a common practice to use fillers in polyolefin compositions. The fillers are used to reduce cost and to impart or enhance a desired property in the finished product required in certain fields of application. Particularly useful are mineral fillers, such as talc, kaolin clay and pyrophylltie. The polyolefin resin and filler are milled together until the filler is thoroughly dispersed. Commonly present in the polyolefins are long-term stabilizers against degradation by oxygen, heat, and light and proecssing aids. The milling process of the filler and polyolefin causes shearing in the resin and an increase in temperature. The result is a tendency to cuase color deterioration of the polyolefin. The polyolefin composition shows a distnict hue, commonly called the "pinking effect". In some instances the pinking effect in filled polyolefins ins increased by the presence of epoxy-type additives and stabilizers of the phenol type. The normal amounts of commmon long-term stabilizers that protect the finished article during its life-time are ineffective to prevent the color deterioration cuased by the filler during the relatively short processing stage, when heat may be generated internally or applied externally. The addition of larger amounts of the stabilizer would be economically impractica and in some instances, detrimental to the overall properties of the product.
Therefore, it is desirable to add to filled polyolefin compositions, a particular stabilizer for reduction of color development, i.e., pinking effect caused by exposure to elevated temperatures during processing and aging.
It is known that talc has a degrading effct on the properties of polyolefins. U.S. Pat. No. 3,553,158 describes the degrading effect on talc on polyolefins and teaches the use of certain "talc deactivators". It further teaches that certain compounds, including zinc dibutylidthiocarbamate are not acceptable because they cause discoloration of talc filled polyolefins during processing or aging. The compoudn, however, are disclosed as long term stabilizers for polypropylene articles containing no fillers in U.K. Pat. No. 929,435.
Surprisingly, it has been discovered that a certin combination of zinc dibutylidthiocarbamate and zinc stearate affords the desired protection of rmineral filled polyolefins during processing and reduces pinking effect in the finished product.